1. Field of the Invention
The invention relates to a module for amplifying a signal light with a remote excitation-light, and further to an optical-fiber communication system including the same.
2. Description of the Related Art
A transmission line in an optical long-range transmission system for transmitting an optical signal by hundreds of kilometers or longer is comprised usually of an optical transmission-line fiber, and an optical relay amplifier for compensating for attenuation of a signal light in the optical transmission-line fiber. A signal light is amplified in the optical amplifier, but receives a noise light while being amplified, resulting in reduction in an optical signal-noise ratio (SNR). Reduction in an optical SNR causes an error in received codes.
Reduction in an optical SNR is dependent on both an intensity with which a signal light is input into an optical amplifier, and how a signal light is amplified. The number of relays and stations in which relays are arranged much influences fabrication and running costs. Accordingly, it is desired to minimize the number of relays, that is, to lengthen a span between relays as much as possible. However, if a span between relays is lengthened without any compensation, an intensity with which a signal light is input into an optical amplifier would be reduced, that is, an optical SNR would be much reduced. Hence, a span between relays is determined, based on an optical SNR required after transmission of a signal light.
Lengthening a span between relays is a key for reducing costs of fabrication of an optical transmission system. Many solutions have been suggested for lengthening a span between relays.
What attracts attention as a solution effective for minimizing reduction in an optical SNR is a distribution Raman amplifying system used in an optical transmission-line fiber. This system is based on Raman scattering in an optical transmission-line fiber. One of advantages obtained by the distribution Raman amplifying system is that an existing transmission-line fiber can be used without any modification thereto, only if a filter for synthesizing a signal light and an excitation light with each other is arranged in a station, that is, an orientation and a wavelength direction of an optical transmission-line fiber are kept as they are.
A solution more effective than a distribution Raman amplifying system for minimizing reduction in an optical SNR is signal-light amplification with a remote excitation-light, in which there is used an optical fiber in which rare earth element is doped and which is used in an erbium-doped fiber amplifier (EDFA) which is a typical optical fiber amplifier into which rare earth element is doped, and an excitation light is transmitted from a remote relay station through an optical transmission-line fiber.
The amplification of a signal light with a remote excitation-light is not for lengthening a span between optical relay amplifiers. However, since a module for amplifying a signal light with a remote excitation-light, arranged in an optical transmission line, is comprised only of passive parts such as an erbium-doped fiber (EDF), it is not necessary to construct a station in which the module is to be installed, for controlling power supply and so on. Accordingly, the amplification of a signal light with a remote excitation-light lengthens a span between relay stations which span is most important for reducing running costs. Herein, passive parts mean parts capable of operating without receipt of electric energy.
Thus, the amplification of a signal light with a remote excitation-light is frequently applied to a field where it is difficult to supply power to an optical relay amplifier, or it would cost so much to do so. For instance, the amplification of a signal light with a remote excitation-light is applied to an optical submarine transmission system used for transmitting an optical signal in a relatively short range, specifically, in about hundreds of kilometers.
Hereinbelow is explained an optical transmission system to which the amplification of a signal light with a remote excitation-light is applied. FIG. 1 is block diagram of a conventional optical transmission system to which the amplification of a signal light with a remote excitation-light is applied, suggested in Optical Fiber Communication (OFC) Conference 2002, Technical Digest, pp. 606–608, paper ThFF1.
The optical transmission system illustrated in FIG. 1 is comprised of a module 303 for amplifying a signal light with a remote excitation-light, a first optical transmission-line fiber 302 optically connecting a first station 301 to the module 303, and a second optical transmission-line fiber 310 optically connecting a second station 308 to the module 303.
The module 303 is comprised of an optical amplifier 304 to which rare earth element is doped and which amplifies a signal light, a first filter 305a for synthesizing a signal light and an excitation light to each other, and optically connected to the optical amplifier 304, a second filter 305b for synthesizing a signal light and an excitation light to each other, and optically connected to the first filter 305a, a bypass circuit 306 through which the first and second filters 305a and 305b are optically connected to each other, and an isolator 307 optically connected to the first and second filters 305a and 305b between them for preventing a noise light from entering the optical amplifier 304.
In the second station 308 are arranged a third filter 305c for synthesizing a signal light and an excitation light to each other, and an excitation-light source 309 which transmits an excitation light by which the optical amplifier 304 is excited is arranged in the second station 308. The third filter 305c and the excitation-light source 309 are optically connected to each other.
A signal light transmitted from the first station 301 enters the module 303 through the first optical transmission-line fiber 302. The excitation light is transmitted to the optical amplifier 304 from the excitation-light source 309 through the third filter 305c, the second optical transmission-line fiber 310, the second filter 305b, the bypass circuit 306, and the first filter 305a. A noise light prevented by the isolator 307 from entering the optical amplifier 304 includes a reflected light resulted from reflection of a signal light in the second optical transmission-line fiber 310, and a light resulted from Raman scattering of an excitation light in the second optical transmission-line fiber 310.
The amplification of a signal light with a remote excitation-light is useful for an extension of a span between relays in a land transmission system, and further for reduction in costs in fabricating and running the system. However, the amplification of a signal light with a remote excitation-light is accompanied with problems. A part of the problems is caused that introduction of the amplification of a signal light with a remote excitation-light, that is, introduction of a module for amplifying a signal light with a remote excitation-light, into an optical transmission system will lose “transparency” of an optical transmission-line fiber.
The first problem is that the present amplification of a signal light with a remote excitation-light has orientation, in other words, the present amplification of a signal light with a remote excitation-light will lose “transparency” relating to orientation. Specifically, if the present amplification of a signal light with a remote excitation-light is introduced into an optical transmission system, it would be difficult for the optical transmission system to operate in an opposite direction. What spoils “transparency” is the isolator 307 as a part of the module 303 illustrated in FIG. 1.
The above-mentioned first problem exerts a harmful influence on a case in which the number of up-link and down-link transmission lines and a ratio of them are determined, taking estimated future demand into consideration, in an optical transmission system in which communication volume tends to be asymmetrical between up and down lines, for instance, in communication between a big city and a small city.
The second problem is caused by that “transparency” of an optical transmission line can be accomplished by an excitation light even in a direction in which an excitation-light amplifier operates. In other words, even if the amplification of a signal light with a remote excitation-light is not always necessary in cases, for instance, where communication volume is small, or a total transmission distance is short, it would be always necessary to introduce an excitation light into an optical amplifier. This is because an erbium-doped fiber (EDF) used for optical amplification would become absorptive medium, if not excited, and hence, a signal light could not pass through the fiber. Preparation of an excitation-light source which is not essentially necessary causes unnecessary increase in costs.
As is obvious in view of the above-mentioned problems, when an optical transmission line to which the present amplification of a signal light with a remote excitation-light is applied is constructed, it would be necessary to in advance determine how the optical transmission line is used. However, it is difficult to do so in an optical transmission line to be used for a land transmission system. This is because since construction of an optical transmission line to be used for a land transmission system costs so much, optical fibers are constructed at a time in number much greater than presently required. Accordingly, when constructed, how optical fibers are used is not determined frequently.
As one of the solutions to this problem, there may be constructed optical fibers without a module for amplifying a signal light with a remote excitation-light, optical fibers with such a module used only for up link, or optical fibers with such a module used only for down link. However, this causes another problem of an increase in optical transmission-line fibers which are not used.
The problem caused when the present amplification of a signal light with a remote excitation-light is applied to an optical transmission line to be used for a land transmission system is that it is necessary to determine how an optical transmission line is used, when the optical transmission line is constructed, and that the constructed optical transmission line has no flexibility for modification or addition thereof. That is, if the optical transmission line is used in other objects than originally designed, modification of the optical transmission line made for enhancing performance thereof might cause an increase in construction and/or running costs.
Japanese Patent Application Publication No. 5-335673 has suggested an optical circulator having a forward-transmission characteristic where a signal is transmitted from a first terminal to a second terminal, from a third terminal to a fourth terminal, and to the first terminal from the fourth terminal, characterized by an optical amplifier arranged between the second and third terminals.
Japanese Patent Application Publication No. 7-212316 has suggested an optical amplifier including first and second optical synthesizer/separators each having first to fourth terminals, and an optical amplifier. In each of the first and second optical synthesizer/separators, the first and third terminals are optically connected to each other for a first wavelength, the second and fourth terminals are optically connected to each other for the first wavelength, the first and fourth terminals are optically connected to each other for a second wavelength, and the second and third terminals are optically connected to each other for the second wavelength. The optical amplifier has an input terminal optically connected to the third terminal of the first optical synthesizer/separator, and an output terminal optically connected to the first terminal of the second optical synthesizer/separator. The second terminal of the first optical synthesizer/separator is optically connected to the second terminal of the second optical synthesizer/separator. The fourth terminal of the first optical synthesizer/separator is optically connected to the fourth terminal of the second optical synthesizer/separator. The first terminal of the first optical synthesizer/separator acts as an input/output terminal of a first optical transmission line, and the third terminal of the second optical synthesizer/separator acts as an input/output terminal of a second optical transmission line.
Japanese Patent Application Publication No. 2001-28569 has suggested a relay in an optical-fiber transmission system including a discrete amplifier operating in accordance with conductive Raman effect, and a residual pumping light is introduced into an input port of the relay in an anti-propagating direction.
Japanese Patent Application Publication No. 2000-151521 has suggested an optical transmission system including an optical fiber through which a signal light is transmitted, an optically amplifying fiber inserted into the optical fiber for amplifying the signal light, an excitation-light source from which an excitation light for exciting the optically amplifying fiber is transmitted, and means for multiplexing said excitation light with respect to a wavelength, and transmitting the thus multiplexed excitation light to the optical fiber.
Japanese Patent Application Publication No. 2002-319726 has suggested an optical amplifier including a first optically-amplifying medium receiving a signal light including a signal light having a first band and a signal light having a second band, and amplifying the signal having a first band, an optical separator which directs almost all lights to a first optical path, and the rest to a second path among lights having been output from the first optically-amplifying medium, a second optically-amplifying medium arranged on the second optical path for amplifying the signal light having a second band, and an optical synthesizer which synthesizes a light transmitted through the first optical path with the signal light having a second band, and outputs the thus synthesized lights to an output terminal.